The Applicability Of Resonant Two-photon Ionization And Pulsed Laser Desorption In Supersonic Beam Mass Spectrometry.

This work investigates the applicability of one color resonant two photon ionization (R2PI) in supersonic beam mass spectrometry. Of particular interest is the fact that photons of one color may not provide sufficient energy to cause ionization even when a laser source is tuned to an excited vibronic molecular state. We have therefore correlated trends in ionization potential with molecular structure for simple systems, specifically, halogenated aniline, phenol and toluene derivatives, mono- and di-substituted benzenes and several azabenzene compounds. Para substituted benzenes were found to ionize efficiently in the absence of ultrafast relaxation processes such as are found in iodo derivatives. However, many of the ortho compounds do not ionize efficiently, presumably due to a combination of dipolar and steric effects which result in an increase in the ionization potential and smaller Franck Condon factors for absorption. These type of effects have been qualitatively related to the electron releasing and withdrawing properties of the substituent groups. The azabenzenes studied were ionized via n (--->) (pi)* transitions and were found to have smaller cross sections. Resonant three photon ionization with possible autoionization from a Rydberg state is believed to occur in these molecules. An extension of this research has involved the use of low power laser desorption for volatilizing nonvolatile and/or thermally labile biological molecules into a supersonic jet expansion with subsequent ionization in a time of flight mass spectrometer using R2PI. The separation of the desorption and ionization steps as compared to direct laser desorption methods allows for the possibility of selective ionization of the neutral fraction desorbed from the surface. The neutral plume is entrained in a supersonic expansion in these experiments thus increasing the degree of selectivity obtained in the ionization process. This technique was used to detect several classes of labile biomolecules including catecholamines, indoleamines, drugs, purine and pyrimidine bases and small peptides. In each case R2PI provides soft ionization with production of only the molecular ion or minor fragmentation. In addition, by increasing the laser energy fragmentation patterns characteristic of the structure of the molecule are obtained.Ph.D.Physical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127985/2/8702843.pd